The present invention relates to an objective lens actuator for an optical head which is arranged in an optical disk apparatus and only reproduces data or can record/reproduce/erase data and, more particularly, to an objective lens actuator, for an optical head, which can cope with high-speed access and uses a lens having a large numerical aperture.
In recent years, studies of an optical disk apparatus which copes with a high-definition system or is designed to digitize a current broadcast system or an external storage apparatus having the high accessibility of a magnetic disk for a computer and the large-capacity storage capability of an optical disk have rapidly advanced. As a next-generation disk apparatus, an apparatus in which the numerical aperture of the objective lens of an optical head, of an optical disk apparatus, capable of recording/reproducing/erasing data, is large, and a laser having a shorter wavelength is used, has been considered to obtain a high transfer rate and large-capacity storage capability. In particular, since the numerical aperture increases and the laser wavelength decreases in a focusing servo operation, a focal depth decreases. For this reason, an objective lens actuator having a high resolution and a high precision has been desired.
In a magneto-optical head which copes with high-speed access, an arrangement of a 1-beam scheme is shown in FIG. 12. A divergent beam emitted from a laser 61 is converted into a parallel beam by a collimator lens 62, and the parallel beam propagates straight through beam splitters 63 and 64 to be incident on an optical path system having different optical lengths corresponding to the inner and outer peripheries of an optical disk. An objective lens actuator 66 having a 45.degree. mirror 65 is arranged at the distal end of the optical path system, and an objective lens is tracked in accordance with the surface vibration or decentering of the optical disk. This tracking operation is performed by signals obtained in focusing error detection (to be described later) and tracking error detection (to be described later) on the basis of a beam polarized by the beam splitter 63.
In this optical system, when an optical disk 67 is radially inclined during rotation, the beam passes through an optical path indicated by broken line in FIG. 12. A beam reflected by the optical disk 67 reversely passes through the above optical path, and the beam propagates along an axis deviating from the optical axis of an original optical design, as indicated by the broken line. The beam is polarized by the beam splitter 63, and is incident on a two-division sensor 68 (to be described later) to be offset from the central axis of the two-division sensor 68, thereby detecting a tracking error signal having an offset.
FIGS. 13 and 14A to 14C show the optical system having the offset in detail. A beam emitted from a laser 71 is converted into a parallel beam, and the parallel beam is polarized by a beam splitter 72 by 90.degree. and reaches an optical disk 74 through an objective lens 73. The return beam propagates straight through the objective lens 73 and the beam splitter 72 to reach a two-division sensor 75. At this time, a differential output of the outputs from the two-division sensor 75 is detected as a tracking error signal by a differential amplifier 76. In this optical system, when tracking error detection is performed while the light beam is offset from the central axis of the two-division sensor 75, the state of the tracking error signal is changed into a tracking error signal having no offset (as shown in FIG. 14A) and into tracking error signals each having an offset (as shown in FIGS. 14B and 14C). Such a tracking error signal having an offset results in an unstable tracking operation.
In the above magneto-optical head, since the length of the optical path between the disk and the position at which tracking error detection is performed must be large, a tracking error offset is impossible. In addition, in order to constitute a polarization optical system for perform magneto-optical detection, the transmittance and reflectance of P and S waves serving as waves polarized by the beam splitter must be controlled. For this reason, an optical arrangement is disadvantageously complex.
In particular, in a magneto-optical head arrangement, a polarization plane must be controlled. For this reason, a head in which the optical arrangement shown in FIG. 12 is directly arranged below an objective lens actuator is considered. However, the polarizing conditions of the beam splitter also used as a 90.degree. rising mirror must be satisfied. More specifically, the transmission efficiency of a P-polarized beam and the reflection efficiency of an S-polarized beam must be optimized. When an S-polarized beam is incident on the beam splitter, it is important to increase the reflection efficiency of the P-polarized beam serving as a magneto-optical signal. In addition, a beam splitter in which the S-polarized beam for detecting a tracking error signal has high transmission efficiency cannot be easily designed.
A conventional objective lens actuator for an optical head must be decreased in size to cope with the trend of a decrease in weight, thickness, length, and size and low cost, the size of the actuator must be small. However, hole portions in which a yoke is inserted must be formed to form a magnetic return path for a closed magnetic path. In addition, the thicknesses of the portions around the yoke insertion hole portions of a lens holder, and focusing and tracking coils which receive a driving force for an electromagnetic drive operation must be large, and the actuator must have mechanical rigidity to prevent generation of a secondary resonance frequency which is undesirable in a servo operation. Therefore, the size of the actuator is disadvantageously increased.
Since a large-size actuator has a heavy weight, a tracking drive sensitivity decreases, and the productivity cannot be easily improved.
In addition, when an open magnetic circuit arrangement is used in place of a closed magnetic circuit arrangement, magnetic utilization efficiency is considerably degraded.